The present invention relates to a method for manufacturing fried tofu products.
"Tofu" or a bean curd is one of the most popular, traditional soybean foods in Eastern Asia and is an important source of protein for people living in this region. It is a watery, white, jelly-like fragile product having a moisture content as high as 88 weight percent. In most cases, tofu is prepared by complicated manual processes. In a typical process, soybeans, as the raw material, are soaked in water for a prolonged period of time, for example, 8 to 24 hours according to the season, and the soaked soybeans are ground to form a slurry thereof. This slurry is heated to extract the soy protein in the form as dissolved in the water phase and then filtered through a coarse cloth to remove insoluble soybean refuse which consists mainly of pulverized cellular substances and is generally known as "okara". The filtrate or soy milk is subsequently coagulated to form a curd by adding a coagulating agent, such as non-toxic alkaline earth metal salt, such as calcium sulfate, calcium chloride or magnesium chloride, and the whey is separated from the curd by draining. In recently developed processes, certain organic gamma or delta lactones of aldonic and uronic acids, such as D-glucono-.delta.-lactone, are used as the coagulating agent and the necessity of the whey draining process is eliminated. Notwithstanding this improvement, the prior art processes used for making the tofu or bean curd still require many laborious, time-consuming procedures and generally are not adaptable for automated mass production.
While tofu itself, either boiled or not boiled, is edible with or without being seasoned, it can be used as a foodstuff in several different forms. In Eastern Asia, it is commonly consumed as a fried product and "aburage" ("age" as it is often referred to for short) is one of the most common forms of fried tofu in Japan.
Aburage is generally square in shape and is prepared by slicing the bean curd into relatively thin slices, pressing the slices to adjust the moisture content to about 80 to 82% and then deep frying in several, for example, two, successive stages. Aburage is consumed chiefly in a cooked state or in a toasted state with appropriate condiments and/or seasonings.
Occasionally, aburage is also consumed as an edible casing or wrapping for other foodstuffs. This aburage casing is prepared by diagonally cutting a square piece of aburage to form two triangular pieces, each triangular piece being then slit open along the cut edge to form a bag. "Inarizushi", a sort of "sushi" popular in Japan, is prepared by stuffing the aburage bag with a vinegared rice. On the other hand, daily dishes can be prepared by stuffing aburage bags with various comminuted vegetables and fastening the stuffed bags with an edible string or cord, such as "kanpyo" made from the fruits from certain convolvulacean and lagenarian plants. Aburage can be cut into two rectangular pieces, each of which is slit open along the cut edge to form a bag which is used in a similar manner.
Another type of fried tofu products is "ganmodoki", also called "hirouse" or "hiryozu", which is prepared by crushing or grinding the drained bean curd, mixing a curd binder, such as dioscorean yams (e.g., D. japonica thunb. or D. betatas decne.), and minced vegetables with the ground bean curd, forming this mixture into a desired shape, and then deep frying the shaped mixture in several, for example, two, successive stages. This product is also consumed in a manner similar to aburage.
U.S. Pat. No. 3,950,550 which issued on Apr. 13, 1976 and the invention of which has been assigned to the same assignee of the present invention, discloses a method for manufacturing a fried tofu product without utilizing such a coagulating agent as used in the traditional, old fashioned but still existing, method hereinbefore described. According to this U.S. patent, a heat coagulable soy protein, for example, a dried or aqueous soy protein isolate, a curd of acid-precipitated soy protein or a soy protein concentrate from which either the soybean refuse or the soy whey has been removed, is used. Specifically, a fried tofu product according to this U.S. patent is manufactured by kneading about 15 to 45 parts by weight of heat coagulable soy protein and 0 to 100 parts by weight of an edible oil or fat together with 100 parts of water with or without the addition of 1 to 10 parts by weight of an egg ingredient, for example, the egg yolk, egg white or both, to provide a plastic-like moldable soy protein dough, shaping the dough into any desired shape, and deep frying the shaped dough in successive stages from under 135.degree. C. to over 135.degree. C.
If the soy protein dough is molded into a sheetlike shape during the shaping procedure, the resultant fried tofu product is aburage.
In view of the fact that the fried tofu product such as aburage can be manufactured directly from the soy proteinaceous material without the formation of the tofu or bean curd which has been required according to the traditional method, the method of the above mentioned U.S. patent involves the following advantages.
(a) The automated mass production of fried tofu products has been enabled.
(b) While the fried tofu products are often desired to be stored in a frozen state for a prolonged period of time, the employment of the alkaline earth metal salt as a coagulating agent such as practised in the manufacture of the traditional tofu products tends to accelerate the denaturation of soy protein when the fried tofu products are frozen. However, the method of the above mentioned U.S. patent does not employ any coagulating agent and, therefore, the fried tofu product manufactured thereby can be stored in a frozen state for a prolonged period of time without being denaturized, is easy to handle and can readily be transported from place to place without being substantially spoiled.
(c) As compared with the fried tofu product manufactured by the traditional method by way of the formation of the tofu or bean curd, the fried tofu product according to the above mentioned U.S. patent has no tendency of losing its shape even when boiled too much, has a moderate, satisfactory mouthfeel or chewiness and can readily be flavored.
Because of these advantages, the fried tofu product according to the above mentioned U.S. patent is largely employed as one of ingredients of "instant" foods and also an accessory to the main dishes, but has been found unsatisfactory and not suited for use in the preparation of the inarizushi which generally requires the use of a bag of a fried tofu product having a relatively firm texture.